I saw this today while testing debian installation in vbox but figured that it existed long before that.
This would be very nice feature to have and it would make alpine setup script even more sophisticated.

I was stunt by that idea to see that in alpine and in thinking on few simple ways to implement that, so time passed and I was too late to create an screenshot. But anyway the results of mentioned feature gave my debian install disabled apt CD paths in sources.list

So I`ve started the search and found some a bit old info on this page:
http://bryars.eu/2011/08/automating-debian-preseed-installs-with-raid-and-lvm/

info:(at the bottom)
——-
Use network install for updates, but cd/dvd for install

d-i preseed/early_command string anna-install net-retriever;

### Mirror settings

1. If you select ftp, the mirror/country string does not need to be set.
   #d-i mirror/protocol string ftp
   d-i mirror/country string manual
   d-i mirror/http/hostname string ftp.uk.debian.org
   d-i mirror/http/hostname seen true
   d-i mirror/http/directory string /debian
   d-i mirror/http/directory seen true
   d-i mirror/http/proxy string

1. Suite to install.
   d-i mirror/suite string squeeze
2. Suite to use for loading installer components (optional).
   d-i mirror/udeb/suite string squeeze

1. disable the cdrom path from the apt soruces.list automatically
   d-i preseed/late_command in-target sed -i ‘s/^deb cdrom/# DISABLED deb/’ /etc/apt/sources.list

(from redmine: issue id 2540, created on 2014-01-07, closed on 2014-05-30)

• Changesets:
  • Revision 1109a9bf by Natanael Copa on 2014-04-23T10:00:10Z:

setup-disk: comment out local repositories

fixes #2540